1. Field of the Invention
The field of the invention generally relates to growing silicon crystals by the Czochralski (CZ) technique. In particular, the field of the invention relates to a system and method for continuous and rapid growing of ultra pure, high minority carrier lifetime mono-crystalline silicon based on improvements to the Czochralski method.
2. Background of Related Art
Referring to FIGS. 1A and 1B, to be useful for the fabrication of semiconductor electronic components, silicon must be formed into a large (about 10-30 cm diameter), nearly perfect, single crystal, since grain boundaries and other crystalline defects degrade device performance. Sophisticated techniques are needed to obtain single crystals of such high quality. These crystals can be formed by either the Czochralski (CZ) technique or the float-zone (FZ) method.
Referring to FIGS. 1A and 1B, in a conventional CZ technique, pieces of polysilicon are first melted in a fused silica crucible 100 in an inert atmosphere (typically argon) within a growth chamber 102 and held at a temperature just above 1412 degrees C., the melting point of silicon. A high quality seed crystal 101 with the desired crystalline orientation is then lowered through pull chamber 106 into the melt 122 while being rotated. The crucible 100 is simultaneously rotated in the opposite direction to induce mixing in the melt and to minimize temperature non-uniformities. A portion of the seed crystal is dissolved in the molten silicon to remove strained outer portions and to expose fresh crystal surfaces.
The seed is then slowly raised or pulled from the melt 122 by a crystal pulling mechanism 108. As the seed is raised, it cools and material from the melt adheres to it, thereby forming a larger crystal or ingot 103. Under the carefully controlled conditions maintained during growth, the new silicon atoms continue the crystal structure of the already solidified material. The desired crystal diameter is obtained by controlling the pull rate and temperature with conventional feedback mechanisms. In this manner, cylindrical, single crystal ingots of silicon are fabricated.
A problem in a conventional CZ process arises when a high temperature charge of molten silicon 122 is heated within a typical narrow diameter, high width, high aspect ratio crucible 100 by means of heater elements disposed around the vertical walls of the crucible. Driving heat though the crucible walls to heat the charge creates stress on the crucible and shortens its useful life. After each growth cycle, the molten silicon remaining in the bottom of the crucible solidifies and expands to such an extent that it can break the crucible. Thus, in a conventional CZ process the crucible is generally a single use item.
The silicon must be continuously heated to remain molten in the crucible. Thus, referring to FIG. 1B, in a conventional high aspect ratio, narrow diameter CZ crucible 100 with heaters 118 disposed around the vertical walls of the crucible, the temperature distribution though the melt is characterized by a high thermal gradient and large temperature difference between the hot walls of the crucible and the coolest spot at the center of the crystal in the solidification zone at the melt/crystal interface as shown at 109. Consequently there is a significant radial temperature gradient and convection velocity gradient across the solidification zone at the melt/crystal interface and the region adjacent to the walls are driven to an undesirably high temperature with attendant excess convection current velocity and thermal perturbations. This condition is sub-optimal for maximized pull rate of high quality defect free crystal. In order to grow more, high quality silicon at a faster rate, a different crucible and heater design is needed that provides a uniform temperature distribution with minimized thermal gradient and convection velocity gradient in the solidification zone at the crystal/melt interface 107.
Conventional CZ grown silicon differs from ideal monocrystalline silicon because it includes imperfections or defects that are undesirable in fabricating integrated circuit devices or high conversion efficiency solar cells. Defects in single crystal silicon form in the crystal growth chamber as the crystal cools after solidification. Defects generally are classified as point defects or agglomerates (three-dimensional defects). Point defects are of two general types: vacancy point defects and interstitial point defects.
In a vacancy point defect, a silicon atom is missing from one of its normal positions in the silicon crystal lattice. This vacancy gives rise to the point defect.
An interstitial point defect occurs when an atom is found at a non-lattice site (interstitial site) in the silicon crystal. If the concentration of such point defects reaches a level of critical saturation within the single crystal silicon, and if the mobility of the point defects is sufficiently high, a reaction, or an agglomeration event, may occur.
In a conventional CZ process, point defects are generally formed at the interface between the silicon melt and the solid silicon. Such defects arise, in part, due to thermal perturbations around the crystal resulting from convection currents and the inability to closely control and or maintain an optimal temperature distribution particularly in the solidification zone at the crystal/melt interface.
Therefore, what is also needed is an improved heating system with multiple separate heating zones to aid in controlling crystal formation rates and defect density. Also, such a configuration should substantially eliminate convection currents and thermal perturbations that lead to the formation of point defects. It also would be desirable to minimize the radiant energy that strikes the crystal during growth, allowing for more rapid cooling of the crystal and higher pull rates. In a conventional CZ process the hottest surface is that part of the crucible wall not submerged in the melt. A high aspect ratio crucible brings this surface in close proximity to the cooling ingot, inhibiting optimal cooling of the ingot largely through heating by radiation.
Another problem with conventional CZ grown silicon is that it contains a substantial quantity of oxygen. This is due to the composition and configuration of the typical high-aspect ratio, narrow diameter crucible, wherein convection currents scrub the walls of the crucible and convey impurities into the melt and ultimately to the crystal itself. The convection currents add oxygen to the melt resulting from the slow dissolution of fused silica (silicon dioxide) on the walls of the crucible holding the molten silicon. This introduction of oxygen into the melt can cause defects in the finished crystal.
In photovoltaic and other applications, high oxygen content in the silicon adversely affects minority carrier lifetime and greatly degrades performance and in photovoltaic devices reduces the conversion efficiency.
Thus, what is needed is a crucible design that can minimize the introduction of oxygen into the melt and provide substantially oxygen free silicon characterized by high minority carrier lifetime for photovoltaic and other applications. The use of a special coating or material for a crucible that would make the crucible resistant to breakdown by molten silicon currently is not feasible since the crucible is a single use item and is broken by solidification of unused silicon during the cool down period after each use.
Therefore, what is also needed is a new crucible design that enables useful crucible lifetime to be extended over many cycles of operation without damage, and thus would make a potentially higher cost inert crucible surface economically feasible. Additional problems with a conventional CZ process are the inability to control dopant concentrations across the melt and across the resulting crystal. For many integrated circuit processes a desired dopant density is added to the silicon. Such dopant concentration is obtained by incorporating a small carefully controlled quantity of the desired dopant element, such as boron or phosphorus into the melt. For accurate control, a small quantity of heavily doped silicon is usually added to the undoped melt. The dopant concentration in the pulled crystal of silicon is always less than that in the melt because dopant is rejected from the crystal into the melt as the silicon solidifies. This segregation causes the dopant concentration in the melt to increase undesirably as the crystal grows. The seed end of the crystal therefore is less heavily doped than the tail end.
The segregation effect is also a function of conditions including temperature. Thus, a non-uniform temperature distribution through the solidification zone, crystal/melt interface provides an undesirable dopant concentration gradient and attendant resistivity gradient along the crystal radius. Accordingly, what is also needed is a simplified crucible design that minimizes segregation and enables dopant concentration and resistivity to be substantially uniform throughout the crystal.